A theoretical study of the isomerization and dissociation processes of the HO–CO+ ion: interstate crossing in the formation of H–CO++O11Dedicated to Professor Mike Bowers in recognition of his impressive contributions to the field of mass spectrometry and ion processes.

Low energy HO–CO+ ions [ΔHf(298) = 142 ± 2 kcal/mol] uniquely dissociate to H–CO+ + O. From ab initio molecular orbital calculations a mechanism is proposed which involves a 1,2-hydrogen shift of HO–CO+ (1A′) to H–CO2+ (1A′) followed by an interstate crossing leading to a transient triplet 3A″ state which then dissociates into H–CO+ (1Σ) + O (3P). This mechanism provides a satisfactory explanation of the observed kinetic energy release and it also satisfies the energetic constraints imposed by experiment. From calculations performed at different levels of theory, it became clear that both an advanced level of electronic correlation and large basis sets are a prerequisite for a meaningful description of the above reaction.